Mining companies grind several thousand tons of ore rocks per days. Generally, the grinding mills used for this task consist of revolving drums which are lined with wear-resistant plates, to resist the abrasion of the ore rocks tumbling against their inside surfaces. These wear plates are replaced periodically when they reach a minimum thickness.
The stopping of one of these mills for inspection represents enormous losses in production time and high energy demand to start it up again. Therefore, there has been numerous efforts made in the past to shorten the inspection time or to reduce inspection frequency to a minimum. For examples;
U.S. Pat. No. 1,772,026 issued to C. O. Bartley on Aug. 5, 1930; discloses a measuring system which consists of one or more screws extending radially through the shell of the mill and into corresponding cavities in one of the wear plates. When the bolt can be inserted further than the bottom of the cavity without resistance, the liner is either broken or has worn to its minimum thickness, and in either cases replacement of the wear plates is in order.
JP Patent 354,035,464 issued to Kubota Ltd., on Mar. 15, 1979, discloses a system wherein the bolts retaining the wear plates have their heads flush with the exposed surfaces of the plates. As a wear plate gets thinner, the bolt heads holding it also wear down. When a bolt head is worn away completely, a spring under the nut of the bolt pulls the nut and the stem of the bolt outside the shell of the mill. The stem and nut fall into a bolt casing and hit a switch in that casing. The switch is connected to a buzzer or other warning device to inform maintenance personnel that a wear plate inside that mill is due for replacement.
Although the systems of the prior art deserve undeniable merits, these systems provide warning when a wear plate is no longer protecting the shell of the mill and the mill must be shut down. These failures can occur during a time of reduced personnel such as a night shift or a holiday. The repair of the mill during these emergency situations is generally done by reduced-size crews and consequently, the mill is unproductive for a longer period than if the repairs would have been carried out during a planned shutdown.
Therefore, there is a need in the industry for a monitoring device which can provide sufficient warning time so that repair work can be schedule ahead, and proper spare parts, tools, rigging and lifting equipment can be brought to the job site in preparation for the job. There is a need for a monitoring device which can provide sufficient warning time to ensure that the repair can be effected quickly, and that the mill can be put back in operation in the shortest possible delay.